Current commercial aircraft typically have two or more propulsion elements, main engines, and an auxiliary power unit (APU). The APU provides electrical or pneumatic energy, or a combination of both, when the aircraft is on the ground. The main engines usually perform this function during other operations. The majority of APUs can also provide energy during flights, and are generally seen as a backup electrical and/or pneumatic energy generation system in the event that either of the energy systems fails.
When the plane is stationed on the ground, the APU supplies electrical and pneumatic energy for the following operations: refueling, passenger boarding and debarking, restocking consumables and provisions for subsequent flights, loading and unloading, cleaning, aircraft maintenance tasks, etc., and to start up the main engines, either electrically or pneumatically.
When the main engines are turned off the APU supplies pneumatic power to the cabin's ventilation and air conditioning system; alternatively the support services on the ground provide energy for this operation. However some cabin ventilation and air conditioning systems can be powered electrically if electrical architecture is installed on the aircraft together with a pneumatic energy generation system independent of a power plant, engines and APU. This system simply requires electricity, which can be supplied by the APU or support services on the ground.
A pneumatic energy generation system can exist independently of an electrical system. The air conditioning system renews the air in the aircraft based on the difference between the selected temperature (for the entire plane or sections of the plane) and the actual temperature inside the cabin. Demand is lower the closer the two temperatures. When they are equal they are said to have reached a steady state condition. The steady state condition corresponds to a constant demand signal as long as all the aforementioned parameters remain steady. It will require an increasingly strong flow the greater the heat in the plane. For example, a pneumatic cabin air conditioning system powered by an APU pneumatic energy supplier typically operates when there is a demand signal of 0% to 100%. When there is a major gap between the selected and actual cabin temperature there is a high level of demand, that is, 100% or close to 100%. As the gap between the two temperatures diminishes the level of demand decreases until it reaches a point of equilibrium in terms of demand, either equivalent to or less than the initial value. The point of equilibrium will be higher the more heat that there has be released (hot days when the temperature in the cabin needs to fall), or the higher the demand for heat (cold days when the temperature in the cabin is required to rise).
The demand signal is sent between the pneumatic air conditioning system controllers and the APU controller.
The cabin temperature can be selected in the cockpit or at different control panels throughout the aircraft cabin and may be adjusted by the cabin crew or maintenance personnel.
When demand increases power increases, independent of whether the pneumatic generator is the APU or the electrically powered pneumatic compressors. Increased demand translates as an increase in the fuel consumption of the power generator. For example if the APU is generating pneumatic power and the demand signal increases by 10%, the level of power the APU has to provide increases by the same measure. This translates as an increase in fuel consumption and an increase in noise generated by the APU. If demand increases 10% the electricity the electrically powered pneumatic compressors consume increases by the same measure. If the APU generates electrical energy, fuel consumption increases, simultaneously increasing pollutant emissions and noise.
An aircraft's ventilation and air conditioning system is designed to provide maximum comfort, even when the aircraft is on the ground. In most cases, maximum comfort requirements fall when the aircraft is stationary.
The technical report indicates there is no device that can control ventilation and air conditioning in the cabin based on environmental variables such as outside ambient temperature, height, or a pre-established limit, to minimize fuel consumption, pollutant emissions or noise.